A bidirectional triode thyristor is commonly referred to as a triode for alternating current (AC) or “TRIAC.” A TRIAC is a three-terminal electronic component that can conduct current in either direction via a current channel when triggered by a current flow into or out of a gate terminal. Once triggered, the device continues to conduct until the current drops below a certain threshold known as the holding current.
TRIACs are capable of controlling large current flow with milliamp-scale gate currents. For AC power control applications, current flow through a TRIAC can be varied by applying a trigger pulse to the TRIAC gate at a controlled phase angle during each AC half cycle. TRIACs are commonly used in low-power motors speed control, lamp dimming, controlling current flow through AC heating resistive elements, etc.
A TRIAC may also be used for current control in DC power applications. In a typical case, the output waveform of a half or full wave rectifier is modulated by a TRIAC firing in response to a phase controlled trigger pulse. Dimming control of LED lighting may employ such techniques. One method of obtaining an analog brightness signal is to apply an analog filter to the waveform chopped by the TRIAC. A digital power controller for lighting dimmers may digitally time sample a rising edge dimmer phase angle by capturing a sharp rising transition within the rectified AC waveform relative to the immediately preceding falling edge. Digital time sampling of a trailing edge dimmer phase angle captures a sharp falling transition relative to the immediately preceding rising edge. Thus, one of two methods is used depending upon the dimmer type, leading edge or falling edge.